1. Field of the Invention
The present invention is directed to an optical system for illuminating and imaging a reflective light valve, and more particularly, to systems using compact lightweight, and foldable optics for personal miniature displays using reflective light valves.
2. Discussion of the Prior Art
Typically, conventional miniature displays, such as head mounted displays (HMDs), are based on miniature cathode ray tube (CRT) or transmission-based liquid crystal light valve technology. The CRT-based systems are bulky, expensive, and heavy, and primarily used for military helmet-mounted applications. This technology is not suitable for lightweight, compact personal displays.
Transmission-based liquid crystal (LC) technology is the preferred technology for these portable miniature displays today. Although appropriate for the low resolution displays currently available, such as sub-VGA to VGA (640.times.480 pixels), this transmission-based LC technology is not adequate for high resolution miniature portable displays. VGA refers to video graphics adapter.
A transmission technology based display requires a clear aperture for transmission of light through the display. A transparent substrate is also required which incorporates all the display driving circuitry (such as active matrix circuitry). Typically, the driving circuitry uses amorphous silicon on glass technology or poly-silicon on quartz technology. The requirements of transparent substrate, clear aperture, and display control circuitry limit the minimum size of the display panel, thus preventing further display size reductions. To achieve smaller size display panels, reflective liquid crystal (LC) light valves are used.
Reflective liquid crystal light valves do not have the size limitation of transmission-based LC light valves. For reflective LC light valves, using crystalline silicon CMOS technology, the active matrix driving circuitry can be fabricated on 10 micron pixel dimensions or smaller. Furthermore, by using reflection liquid crystal devices, the requirement for a clear aperture in the display panel, needed for transmissive LC devices, is dispensed with. Instead, the reflective device incorporates a mirror array that is fabricated over the underlying CMOS circuitry. In this case, the entire surface of the device is available for display aperture. Thus, the pixel size is only limited by the CMOS technology required to fabricate the drive circuitry, which today is less than 10 microns per pixel. The functioning reflective display panel is completed when the liquid crystal and top glass are assembled over the mirror array.
Thus, miniature high resolution (&gt;VGA) displays can be fabricated using silicon-based reflection liquid crystal devices. However, reflection-based light valves, such as liquid crystal (LC) spatial light modulators (SLMs) have complex illumination requirements. In reflection mode, the SLM must be illuminated and imaged from the same side. A simple backlight structure typically used in transmission-based displays is not directly applicable for reflective SLMs.
In order to illuminate the reflective SLM with polarized light, and image the SLM using a perpendicular polarization, typical optical systems incorporate a polarizing beam splitter cube (PBS) over the SLM.
FIG. 1 shows a conventional optical system 10. A light source 12 illuminates a reflective SLM 14 through a PBS 16. Image forming light, which is reflected from the SLM 14, passes through the PBS 16 and is viewed through an optical imaging system 20. The optical imaging system 20 has several lens elements, such as lens elements 22, 24.
The PBS 16 receives polarized light from the light source 12, passes one polarization, e.g., p-polarization, and reflects the other polarization, e.g. s-polarization. The p-polarized light beam 26 passing through the PBS 16 is incident onto the SLM 14 at largely normal incidence to the SLM 14.
The liquid crystal SLM 14 functions by selectively rotating the p-polarized light beam 26 to s-polarized light beam 28 at the individual pixel level to form an image in the SLM 14. The p-polarization of light (not shown) reflected from the SLM 14 passes through the PBS 16 and is discarded. The s-polarized light beam 28 reflected from the SLM 14, which is the image forming light resulting from selective polarization rotation by the SLM 14, is reflected by the inner surface 30 of the PBS 16 and directed toward the optical imaging system 20. Next, the image forming light 28 is imaged by the optical imaging system 20 to provide the proper imaging of the SLM 14 to a viewer 32. The illumination is thus incident onto the SLM 14 through the PBS 16.
A typical light source for miniature liquid crystal displays (LCDs) uses cold cathode fluorescent light sources (CCFL). One example is a linear CCFL tube coupled to a flat backlight structure. This example is a miniature version of the backlight that is typically used for conventional LCD laptop computer displays. Another example is using a CCFL source that is itself flat and rectangular. Both examples produce a compact flat surface emitting light source. The light source 12 depicted in FIG. 1 is a typical CCFL-based backlight (either flat CCFL or backlight panel incorporating a linear CCFL tube).
The angular distribution of light emitted from backlights is typically larger than the acceptance angle of the LCD. The addition of light brightness enhancing polymer films improves the directionality of the light, but cannot produce a collimated light source. In FIG. 1, a collimating film 35 and an optional lens 40 are shown located between the backlight 12 and PBS 16, respectively. The collimating film 35 and optional lens 40 collimate light from the backlight 12, and direct the collimated light to the SLM 14 through the PBS 16. The collimating film 35 is disposed on the backlight surface that faces the lens 40. The lens 40 is used for focusing and directing the light from the collimating film 35 to the PBS 16.
Although the conventional optical system 10 provides useful illumination to the SLM 14, the optical system 10 is not optimal and suffers from a number of disadvantages. First, light coupling to the SLM 14 is inefficient. Second, there is no control for the numerical aperture (NA) of the illumination.
Even when used with the collimating film 35 and the focusing lens 40, the angular distribution of the light entering the PBS 16 from the backlight 12 is larger than the acceptance angles of the PBS 16 and SLM 14. The polarization of the light beyond the acceptance angles is not adequately controlled by the collimating film 35 and/or focusing lens 40. This produces poor contrast in the resulting image. Furthermore, light at the extreme angles will scatter off the numerous optical surfaces producing additional depolarized background stray light and ghost images that will further degrade the image contrast.
In order to provide an efficient well-controlled illumination to the SLM, relay optics and an illumination aperture stop are included. FIG. 2 shows such a conventional illumination system 50. The illumination system 50 includes multi-element relay optics 52 to couple light from the light source 12 to the SLM 14. In addition, the illumination system 50 includes an illumination aperture stop 54 in order to control or limit the numerical aperture or angular distribution of light.
As in the conventional illumination system 10 of FIG. 1, in the conventional illumination system 50 of FIG. 2, the illumination is incident onto the reflective SLM 14 through the PBS 16. The light source 12 is imaged onto the SLM 14 by the multi-element relay lens 52, which has several optical elements, such as lenses 56, 58, 60, 62. The aperture stop 54 is within the multi-element relay lens 52, and is used to limit the numerical aperture of the illuminating light. The light source 12 itself incorporates the collimating film in order to enhance throughput. FIG. 2 shows the collimating film 35 located on a surface of the backlight 12 that faces the multi-element relay lens 52.
Although the conventional illumination optical system 50 is adequate for illuminating the reflective SLM 14, the optical system 50 is large and bulky. In addition, the optical system 50 is not suitable for portable personal displays, particularly compact, lightweight, head mounted displays.